While geofoam is approximately 1 to 2 percent of

the weight of soil and 10 percent the weight of water, its

compressive strength makes it suitable for many structural

applications. It can be made in many shapes and sizes, but comes

standard in 4-by-8-foot blocks.
PHOTOS BY NOAH SMITH While geofoam is approximately 1 to 2 percent of the weight of soil and 10 percent the weight of water, its compressive strength makes it suitable for many structural applications. It can be made in many shapes and sizes, but comes standard in 4-by-8-foot blocks.
Geofoam can be

an especially attractive material for large waterfeatures. They

remove much of the extra surcharge load on the shell and can be

easily stacked and sculpted into any shape. A thin shell is then

shot over it.
Geofoam can be an especially attractive material for large waterfeatures. They remove much of the extra surcharge load on the shell and can be easily stacked and sculpted into any shape. A thin shell is then shot over it.

With the economy struggling in recent years, architects, designers and contractors have continually searched for ways to make pool construction more economical. Subsequently, projects have realized cost savings by utilizing innovative and efficient construction means and materials. Geofoam has been one such solution.


Geofoam is a rigid, engineered, lightweight fill material made up of expanded polystyrene (EPS) or “foam”. Geofoam is approximately 1 to 2 percent the weight of soil and 10 percent the weight of water. Yet, geofoam maintains a compressive strength suitable for many structural applications making it an attractive fill material. Although geofoam can be manufactured in many sizes and shapes, a standard block is typically 4 feet wide by 8 feet long. Once on site, geofoam can be easily trimmed to the required size by using a hot wire cutter, hand saw, or chain saw. Due to the low density of geofoam, the blocks can be maneuvered by hand or placed with small mechanical equipment. Geofoam is typically placed on level ground with the first course sitting on 2 inches of sand. When placed, the blocks are staggered so their joints are not located on the same vertical plane, similar to kids’ building blocks. To prevent shifting, the blocks are interconnected with either barbed gripper plates or an approved adhesive. Recent costs estimates of geofoam vary from $50 to $100 per cubic yard depending on the required density needed.


Retaining/Pool Walls — The design and construction of pool walls are generally driven by the lateral pressure from soil backfill. The amount of pressure placed on the walls is directly proportional to the weight of the backfill material. As a result, lateral pressures on a pool wall can nearly be eliminated by replacing the soils within the “active” zone behind the wall with geofoam. This can be accomplished by laying back the native soil or backfill to its natural angle of repose. The angle of repose is the maximum slope at which loose, solid material will remain in place without sliding. The resulting wedge shaped void behind the wall is then backfilled with geofoam. Similarly, the use of geofoam as backfill nearly eliminates the lateral earth pressures on a wall generated from a seismic event. By reducing the lateral pressures on a wall, designers can specify lower structural requirements for the wall and foundation leading to significant cost savings on materials and labor.

Rooftop Landscaping — Green roofs for mechanical rooms and cabanas have become more desirable in recent years and are found on a large percentage of new construction projects. Some projects include rooftop planters while other roofs are covered with several feet of soil for extensive planting. Regardless of the design, geofoam can replace traditional soil or gravel backfill above the roof significantly reducing vertical dead loads thus allowing the designer to minimize the structural requirements of the roof and supporting foundations. The ease of block placement over rooftops also makes geofoam more attractive than the alternatives. Sufficient planting soil can still be placed around the geofoam to maintain desired landscaping. Placing geofoam over rooftops also provides an added insulating benefit.

Insulation — As energy costs continue to soar, it is becoming more and more expensive to heat swimming pools and maintain spa temperatures. One of five ways that pools lose heat is through conductance to the soil surrounding the shell. One method that can help minimize this type of energy loss is to insulate the pool with geofoam. Given that it is approximately 98 percent air by volume, geofoam is a very efficient insulator. Significant energy savings are achievable simply by lining the pool/spa excavation with 2 to 3 inches of geofoam prior to gunite/shotcrete placement.

Compressibility — Swimming pools are sometimes constructed in multi-story buildings or over below-grade basements or parking garages. In this instance the pool is constructed within a concrete vault designed by the building structural engineer. It is common for the vault supporting the pool to sag and deflect over time. Since a pool shell is a relatively rigid structure, it is prone to cracking if such deflections are experienced. A cracked pool leaking water where habitable space is located below is not a very desirable situation. As an option to help reduce the potential for damage caused by vault deflection, geofoam can be placed between the floor of the concrete vault and the floor of the pool. As the underlying vault floor sags and deflects, the geofoam compresses and re-distributes the loading from the pool and reduces the amount of deflection the pool actually sees. Geofoam is available in a wide selection of compressible resistances that can be selected based on the anticipated load and deflection.

Waterfeatures/Slides — Large waterfeatures and waterslides are constructed in conjunction with swimming pools on a regular basis. Waterfeatures are traditionally constructed using various methods such as solid sculpted concrete, stacked natural rock, and glass-fiber reinforced concrete (GFRC) panels. All of these options require large amounts of material that have considerable weight. When constructed adjacent to a swimming pool, these features can place large loads on the pool shell. Subsequently, the pool shell has to be strengthened in order to support the additional surcharge loads. As an alternative to traditional construction, the majority of the feature could be made using geofoam.

Once the geofoam is stacked into the desired configuration, a thin structural shell can be shot directly over it (no forms necessary) and can be sculpted into rock or shaped for natural rock placement. Water features constructed in this manner weigh only a fraction of those created with traditional methods and significantly reduce surcharge loads imposed on adjacent pool walls. Similarly, geofoam can be stacked along the underside of a proposed waterslide. Shotcrete can then be shot over the geofoam and formed into your desired waterslide size. The geofoam acts as a permanent interior form and reduces surcharge loads on the pool shell due to the weight of the completed slide.


The use of geofoam on a project is only limited to one’s imagination and creativity. The unique properties of geofoam allow impossible projects to be possible. For example a recent pool was designed as a 6-foot-deep vessel with a 6-foot-tall raised bond beam. The raised bond beam retained a flat backfill that had natural landscaping. Several months after the pool was completed, the owner decided that he wanted to construct a 5-foot-tall retaining wall immediately behind and above the raised bond beam to create an elevated lawn area.

The problem was the raised bond beam and pool wall were not designed to support the additional surcharge from the retaining wall and additional 5 feet of soil. The owner’s request would have been impossible had it not been for the innovative use of geofoam. In lieu of placing an additional 5 feet of soil behind the proposed retaining wall, 5 feet of geofoam was placed instead. By using geofoam, nearly all potential surcharge loads from the fill were eliminated, making the project a reality. Moreover, the structural requirements of the retaining wall were minimized and the fill was able to be placed by hand without the use of mechanical equipment as the original pool construction cut off easy access to the site.


Using geofoam as an alternative to traditional soil fill or forms of construction has many benefits that lead to overall project cost savings. The following are some of the added benefits of utilizing geofoam:

  • Shotcrete, gunite, or soil can be placed directly against geofoam eliminating the need for expensive forming.
  • Construction traffic and import costs are minimized as 1 flat-bed truck of geofoam is equivalent to 12 to 13 dump truck loads of traditional fill. Congestion on the roads leading in and out of a project site is minimized.
  • The construction time of a project is reduced as several feet of geofoam can be placed in a fraction of the time that it would take to place and compact traditional soil fill in the required 8- to 12-inch lifts. As an added incentive, geofoam does not require compaction testing like traditional fill.
  • Tight construction scheduling can easily be maintained using geofoam as it can be installed during any type of weather or site conditions.
  • The lightweight nature of geofoam allows it to be maneuvered by hand and used on projects with tight construction access where the use of larger mechanical equipment may not be feasible.
  • Geofoam is eco-friendly. It can be recycled and reused in many additional manufacturing processes.


As with any construction material, there are also special design considerations that must be acknowledged where geofoam is used in design:

  • Geofoam is subject to damage when exposed to certain hydrocarbon chemicals or solvents. The presence of these items should be considered during construction and if needed, the geofoam can be blanketed with hydrocarbon resistant geomembranes for protection.
  • Although geofoam is treated with fire retardant, the EPS used to manufacture geofoam is combustible. Care should be taken when construction around geofoam includes open flames.
  • Geofoam exposed to sunlight for extended periods of time is susceptible to degradation from ultra-violet light although the degradation does not tend to hinder the product’s integrity. Surficial discoloration generally occurs but can be removed by washing or grinding.
  • Given the lightweight nature of the geofoam blocks, care should be taken when stockpiling the material on job sites where windy conditions exist. The block stockpiles can be weighted or tied down as necessary.